Does low MCV or low hgb cause inability to concentrate in class?

Anemia. You are correct. Anemia requires a work up and treatment.
Probably not. It all depends on how low the hemoglobin is. If it is above 10 i doubt any of this is effecting your concentration. However if it is due to a vitamin B12 deficiency, this can cause problems.
Depends. What you are talking about has to do with your red blood cell counts or rbcs. They carry oxygen to your tissues. If your rbcs are too low, that's call anemia. There a few kinds of anemia but we need more information to advise you what you might needs. Commomly, rbcs low as a consequence of iron deficiencies. There is now a simple way to determine the oxygen in the blood. See the doctor.